The invention concerns a method for treatment of raw brines from desalination plants with a total salt content of greater than 60 g/l as well as a device for performing such a method.
It is known to employ seawater for producing drinking water. For this purpose, the seawater is separated in a seawater desalination plant into freshwater and into raw brine. A known method for desalinating seawater is for example reverse osmosis (RO). The salt content of seawater is lowered with such a method from approximately 35,000 ppm to a total salt content of less than 500 ppm. The reverse osmosis (RO) is a membrane method that operates with pressure. The osmotic pressure of seawater is approximately 25 kg/cm2. When seawater is pressed in a reverse osmosis system at a pressure higher than its osmotic pressure through the membrane, the seawater is separated into freshwater (permeate) and saltwater (concentrate). The higher the total salt content, the higher the osmotic pressure. High pressure pumps consume a lot of energy and represent a significant cost factor in producing freshwater. In conventional seawater reverse osmosis methods (SWRO), from the supplied water approximately 42% freshwater are obtained while 58% concentrate remains. The concentrate (raw brine) has a total salt content of approximately 60,000 ppm (almost 1.7 times the salt content of seawater). A more effective desalination is difficult to achieve not only due to pressure problems but also because of the problems of clogging of the membrane with gypsum and other hardness components of the seawater. These limitations cannot be prevented even with an acid pretreatment of untreated seawater and addition of chemicals for prevention of deposits.
A further method for producing drinking water from seawater is thermal distillation. The most common distillation methods include the multi-stage flash evaporation (MSF), the multi-effect distillation (MED), and vapor compression (VC). In MSF, the supplied water is heated and pressure is reduced so that the water turns to vapor in a flash. This process represents one stage of several serially connected stages of which each is at a lower pressure. In MED, the supplied water is passed through several evaporators that are connected in series. The vapor of one row is subsequently used in order to evaporate the water in the next row. The VC method comprises evaporation of the supplied water, compression of the vapor, the subsequent use of the heated compressed vapor as a heat source for evaporation of further supplied water. Some distillation plants are a mix of more than one desalination technology. The waste product of these processes is a solution with a high salt concentration (raw brine). In conventional thermal distillation processes from the supplied water less than 47% of freshwater are obtained while more than 53% of concentrate remains. The concentrate has a total salt content of approximately 65,000 ppm (almost 1.8 times the salt content of seawater). These limitations are caused by the generation of deposits on heating surfaces of evaporators from hardness components of the seawater, in particular gypsum. Because of the anomalous effect of the reduction of the gypsum solubility in hot solutions, this limitation is often referred to as the “gypsum barrier”.
None of the aforementioned industrial desalination methods is an environmentally friendly technology and they cause enormous contaminations in the marine animal and plant world. All desalination facilities in the world together introduce annually approximately 9 cubic kilometers (9,000,000,000 m3/y) of concentrates without treatment directly into the coastal areas of the oceans which leads to an ecological imbalance. Moreover, there are economic disadvantages. For example, the raw brine that is discharged into the ocean contains large quantities of valuable components such as magnesium, sodium, potassium, and rare metals that are not utilized.
In order to reduce these problems, several methods have been developed which limit the proportion of the produced raw brine. U.S. Pat. No. 6,508,936 discloses a combined method for seawater desalination in order to obtain a very high yield of freshwater. In the method, nanofiltration as a first desalination step is used in combination with thermal distillation, such as the multi-stage flash evaporation (MSF) or the multi-effect distillation (MED). A disadvantage of this method is however that the nanofiltration step is relatively expensive for the purpose of lowering the seawater hardness so that in the step of thermal distillation subsequently more freshwater can be obtained. Moreover, with this method raw brine is also generated.
A method for seawater desalination without producing a raw brine that cannot be processed further is disclosed in WO 2007/132477. In this method, the seawater is first subjected in a pretreatment step to nanofiltration wherein preferably bivalent ions are removed. The removal is approximately 85% per passage and, moreover, not more than 30% of the monovalent ions reach the retentate. This retentate with a high proportion of bivalent ions is utilized for obtaining magnesium and other bivalent ions. The permeate that is containing substantially no bivalent ions can be subjected to a three-stage high-pressure desalination with reverse osmosis (HPSWRO) for producing freshwater. The high-purity brine (HPSWRO concentrate flow with a total salt content of more than 85,000 ppm) can be used by means of electrolysis for obtaining sodium hydroxide, chlorine, and hydrogen. This method has however a few disadvantages. The nanofiltration and the multi-stage HPSWRO require an additional energy expenditure and are also cost-intensive. Moreover, in practice there are significant difficulties with a thorough and separate removal of magnesium and calcium from the nanofiltration concentrate.
In view of the afore described prior art, the invention has the object to provide a method with which raw brine from desalination plants with a total salt content of greater than 60 g/l can be operated with little energy input and cost-efficiently.